1. Field of the Invention
The invention relates to unsaturated polyester resins, to methods of making them, and in particular to the inhibition of evaporation of styrene monomer therefrom.
2. Description of the Prior Art
It has been known for many years that the inclusion of small quantities of paraffin wax in polyester resins prevents air inhibition of the open surface of mouldings on cure (British Pat. No. 713,332). The wax works by migration to the open surface forming a thin layer which prevents air from interfering with the surface cure. The quantity needed depends on the melting point of the wax used, commonly a paraffin, although other waxy materials have been claimed, e.g. stearyl stearate (British Pat. No. 850,762). In practice about 100 ppm. of a paraffin wax mp 52.degree.-54.degree. C. has been employed.
More recently a need has arisen on environmental grounds to reduce the evaporation of styrene monomer from the surface of a laminate in the interval between lay-up and cure. This can be achieved by the incorporation of higher levels of paraffin wax, and the effect of different levels of wax on the styrene loss is progressively greater as the levels of paraffin wax increase up to about 800 ppm. for most types of resin, although some types will require even more wax.
However, at the higher levels of wax required to substantially reduce styrene evaporation several drawbacks have been noted:
(i) the wax can separate from the liquid resin when it is stored at low temperatures,
(ii) the layer of wax can interfere with the adhesion between one layer of laminate and the next, especially where there is a delay between lay up and cure.
Several method of overcoming these disadvantages have been proposed i.e. the incorporation of small quantities of a straight chain hydrocarbon, e.g. n-octane or the use of a surfactant such as polyoxyethylene sorbitan fatty acid esters (German Offenlegungsschrift No. 2554930).
Neither of these methods has been particularly successful. For example 2% n-octane lowers the heat deflection temperature of the cured resin and imparts a milky appearance to the laminate.